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Different tungsten oxide nanocrystals were synthesized via facile hydrothermal process – one-step and free of additives - at different reaction temperatures and a highly acidic environment. The phase transformation of samples, followed by the change of morphology and optical properties, was observed as the reaction temperature varied from room temperature to 220oC. The crystal phase transformed from monoclinic WO3∙2H2O to orthorhombic WO3∙H2O, then to monoclinic WO3 as the reaction temperature increased from room temperature to 100 ⁰C, then to 220 ⁰C. Corresponding to the phase transformation, the optical bandgap increased from 2.43 eV to 2.71 eV, and the morphology varied from nanoplate to nanocuboid. The effect of the reaction temperature on the phase transformation was assigned to the dehydration process, which became stronger as the reaction temperature increased. These results gave an insight into the phase transformation and implied a simple method for manipulating the crystal phase and morphology of tungsten oxide nanostructure for various applications.

In situ nanocomposites of graphene (Gr) and tungsten oxide hydrate (WO[Formula: see text]O) nanoplates were synthesized via facile and energy-saving acid precipitation method at room temperature. The effect of the in situ composition on the physicochemical properties of WO[Formula: see text]O platform material in nanocomposites was examined with X-ray diffraction, Raman scattering, UV–Vis reflectance spectra. Gr helped reduce microstrain, reduce the average crystalline size, increase the distance between (111) planes in WO[Formula: see text]O platform nanoplates, and abnormally increase the optical bandgap. All samples showed a moderate photocatalytic activity in methylene blue (MB) removing efficiency under visible light irradiation, which was remarkably enhanced by adding 0.1[Formula: see text]mL H2O2. In comparison with both pristine and nanocomposite samples, the sample having 0.1[Formula: see text]wt.% of Gr in nanocomposite showed the highest MB removing efficiency of 47% and 99% in corresponding to without and with 0.1[Formula: see text]mL H2O2. The photocatalytic activity was assigned to the key role of the photogenerated holes, which was enhanced by compositing with Gr and/or the appearance of H2O2. The results of this work were a contribution to studying the photocatalytic mechanism of tungsten oxide hydrate-based nanocomposite and also proposed a facile method for preparing WO[Formula: see text]O-based nanocomposite.

This work reported the abnormal constraint effect of graphene on the structural and optical properties of in situ grown Gr@WO3 nanobrick (NB) hybrid nanomaterials. Stable hybrid Gr@WO3 nanomaterials were synthesized through a one-step hydrothermal method, in which different graphene contents (0.1, 0.3 and 0.5 wt%) were directly introduced into the precursor solution of tungsten oxide. The existence of graphene in the nucleation phase of tungsten oxide NB decreased the crystallite size from 49.5 nm to 45.3 nm, increased the microstrain from 1.10% to 1.44% and the optical bandgap of the hybrid samples from 2.68 eV to 2.78 eV, and reduced the radiative recombination efficiency of tungsten oxide NB. The hybridization between graphene and tungsten oxide NBs was confirmed by X-ray diffraction and Raman scattering analysis. The optical properties of materials were studied based on the absorbance and diffuse reflectance spectra. In this work, we also proposed a reliable method to estimate the optical bandgap of the sample from the diffuse reflectance spectrum. These findings would introduce an alternative method of fabrication and enhance the understanding of in situ hybrid materials.

Reconstruction and stabilization of polar oxide surfaces, such as ZnO, contribute a significant role in photocatalysis, chemical sensing, and optoelectronic applications, however their physical chemistry insight is still a puzzle in the surface science. In this work, the polar surface instability induced the morphological evolution of hydrothermally synthesized micro-rod ZnO doped with various contents of Cu2+ ion (1-10 at.%) was investigated. The transformation of micro-rod morphology from the high aspect ratio flower-like shape of the pure ZnO to the hexagonal prism-like shape of the doped ZnO was characterized by X-ray diffractometry, scanning electron microscopy and micro Raman spectroscopy. The chemically active Zn-terminated polar surface in doped samples was less positive charge density which was the main reason to cancel the electrostatic instability for the dominant growing direction. Furthermore, the schematic models of the electron transferring from the conduction band region to the electron trap centre of Cu2+, and the Zn-terminated polar surface reconstruction were proposed for the morphological evolution mechanism.

This study systematically investigated the effects of the acidity of precursor suspension on the formation, physical properties, and adsorption activity of tungsten oxide nanostructures prepared via the acid precipitation method at room temperature. In highly acidic suspension (−0.85 ≤ pH ≤ −0.34), nanoplate orthorhombic tungsten oxide dihydrates (WO3·H2O) with a high crystallinity but with low adsorptive activity were formed. In moderate acidic suspension (0.04 ≤ pH ≤ 0.97), a novel hexagonal tungsten oxide, H-WO3, formed with a mixed morphology and a low crystallinity but with a high adsorptive activity. The effect of acidity was due to the vital role of high H+ ion concentration platform that acted as a scaffold for long-order arrangement and a breaker for confining the nanostructure of tungsten oxide. H-WO3 samples exhibited a high adsorption capacity (~30 mg/g) and a short adsorption–desorption equilibrium time (~15 min) with methylene blue. The adsorption kinetics of the samples followed the pseudo-second-order model, whereas their adsorption isotherm followed the monomolecular adsorptive Langmuir model. This study provides a simple energy-saving method for controlling the crystal structure and morphology of tungsten oxide nanostructures.

Nanocomposite materials of stoichiometric WO3 nanobricks and commercial carbon nanotubes (CNTs) with different contents (0.5, 1.0 and 1.5 wt%) were simply fabricated by dispersing precursors in dimethylformamide. Stoichiometric WO3 nanobricks were directly synthesised by a hydrothermal method without any post-annealing treatment. At low temperatures, the nanocomposite-based sensors showed synergistically enhanced ammonia gas-sensing properties in comparison with CNT-based and WO3 nanobrick-based sensors. The 1.0 wt%-CNT-based sensor showed the most effective enhancement with 28-time higher response and 2-time shorter recovery times than the pristine CNT-based sensor. The limit of detection of the 1.0 wt%-CNT-based sensor could reach sub-ppm level detection (i.e. 150 ppb). The mechanism of this synergistic enhancement in composite materials was assigned to the hole transfer between the CNT surface and the inversion layer on stoichiometric WO3 nanobricks.